The 'Stiches' hitmaker - who previously stripped off for a Calvin Klein campaign - revealed he suffered with "body dysmorphia through trying to be that person all the time".
Speaking to Wonderland magazine, he explained: "I was shirtless on a billboard. Any shoot where you're basically 'sexy' in any way can really mess with your psyche because you're struggling every day to live up to that guy.
"The lighting was there, you have makeup and a hairstylist there. People are there to make you look amazing.
But realistically, you can't really fit the shoes of that person in that photo - even as that person.
Shawn - who has now posed with just an acoustic guitar covering his modesty for the publication - noted that comparing yourself to other people isn't "fair to anybody".
He added: "It's interesting because I know that there's a lot of struggle in comparison culture.
"It's important for me to say that even as the person in the photo, I cna't be the person in the photo.
"So, striving to be that person is not fair to anybody."
Meanwhile, the 22-year-old singer - who is dating Camila Cabello - admitted he warned his partner not to get too excited after the photo shoot as he insisted he couldn't "live up to that guy".
He said: "I ran down from the shoot being like, 'Damn, these photos are really awesome but I can't live up to that guy!'
"And that was the first thing I told her and she understood that completely."
Now, Shawn has been going to therapy two times a week - while trying "different types" - to help overcome his struggles.
He explained: "You have to be really open and okay with being messed up sometimes.
"That's when you start to make serious progress."
Credit: Bang Showbiz